While early work on varifocal lens systems referred to three and even two element optically compensated systems, subsequent varifocal lens systems have become increasingly complex involving large groups of lenses. Prior art optically compensated three element varifocal lens systems, moreover, consisted of a front lens coupled to a back lens and a fixed center lens. The front and back lenses were both negative or positive lenses and were longitudinally shiftable along the optical axis of the system from the wide angle to the telephoto position. Where the front and back lenses were positive the fixed center lens was negative. Where front and back lenses were negative the center lens was positive; F. G. Back and H. Lowen, Journal of the Optical Society of America, Vol. 44, No. 9, pp. 684-691, September, 1954; F. G. Back and H. Lowen, Journal of the Optical Society of America, Vol. 48, No. 3, pp. 149-153, March, 1958; T. H. Jamieson, Optical Acta, England, Vol. 18, No. 1, pp. 17-30, 1971.
In the varifocal lens system of the present invention the front and back lenses are substantially fixed while the inner lens is axially shiftable to change from the wide angle to the telephoto position. Moreover, the front lens is a negative lens while the back lens and inner lens are positive. Such a lens system provides advantages in improved light transmission, compact size, light weight, large back focal length and is more trouble free is its operation as hereinafter more fully set forth.